1. Field of the Invention
The present invention is directed to a rotating anode X-ray tube, and in particular to a rotating anode X-ray tube of the type having a deflector that interacts with the electron beam proceeding from the cathode to the anode to influence the position of the focus on the anode.
2. Description of the Prior Art
In the case of a rotating anode X-ray tube, it is known to provide a vacuum housing wherein the rotating anode is mounted in a larger volume of the housing, and the cathode is disposed in a generally cylindrical projection of the housing, having an interior that communicates with the larger volume in which the anode is disposed. The cylindrical projection is connected to the larger volume of the housing by a neck having a reduced diameter compared to the cylindrical projection in which the cathode is disposed. The electron beam emitted by the cathode proceeds through this neck and strikes the anode at a focus, from which an X-ray beam emanates.
It is also known in a rotating anode X-ray tube having a housing configured in this manner to dispose a deflector, typically an electromagnetic deflector, at the exterior of the housing at the neck region. Such a deflector typically has a U-shape, with two generally parallel legs that straddle the neck region of the housing. This deflector is controlled to generate a magnetic field of a selected strength, to deflect the electron beam propagating from the cathode to the anode in a defined manner, so as to influence the position of the focus on the anode.
A rotating anode X-ray tube of the type described above is disclosed in U.S. Pat. No. 5,909,479.
The U-shaped beam deflector (yoke) is typically manufactured of stacked laminations, to reduce eddy current losses. Because of this laminated structure, the yoke cannot be effectively evacuated, and therefore it cannot be placed inside of the evacuated housing of such a rotating anode X-ray tube. This has conventionally necessitated its placement as described above, straddling the exterior of the neck region. The physical dimensions of the neck region thus impose a minimum distance between the electron beam and the deflector, which cannot be reduced in a conventional rotating anode tube of the type described above, due to the necessity of keeping the deflector at the exterior of the housing. In order to increase the effectiveness of the deflector, and allow the use of a magnetic deflection field having a lower field strength, therefore requiring less current to be supplied to the electromagnetic coil that generates the deflection field, it would be desirable if the deflector, or at least the legs thereof, could be disposed closer to the electron beam itself.
One possible way to accomplish this would be to make the deflector, or at least the legs thereof, out of solid material, rather than a stack of laminations, so that the evacuation problems associated with the laminate structure would be avoided, and thus the deflector could be placed inside the evacuated housing. As noted above, however, the laminate structure serves a beneficial purpose, namely minimizing eddy current losses, and therefore abandoning the laminated structure would be degrade the operation of the deflector.
Another possibility to reduce the distance between the deflector and the electron beam would be to make the neck region of the evacuated housing smaller in diameter. X-ray tubes, however, must be able to withstand relatively rugged usage, and therefore the neck region of the evacuated housing must have a certain size in order to provide the necessary mechanical stability to avoid a fracture at the neck region during usage of the X-ray tube. Rotating anode X-ray tubes typically are contained within a radiator housing that is filled with a coolant, such as oil. A fracture of the evacuated housing of the X-ray tube at any location is a serious problem, because not only is the vacuum then destroyed, precluding further operation of the X-ray tube, but also oil contained in the radiator housing can then leak into the interior of the evacuated housing.